1. Field of Use
This invention relates to synchronizing operations and more particularly to synchronizing operations taking place within two independently operated systems.
2. Prior Art
In certain situations, it becomes desirable to be able to interconnect two independently operated bus systems for exchanging information therebetween. In such systems, it is possible to encounter deadlock conditions where units on both bus systems attempt to communicate with each other in close time proximity through the same communication path. To avoid this situation, one system has used an intersystem link (ISL) logic unit for controlling the bidirectional transfer of information requests. This type of unit is described in U.S. Pat. No. 4,234,919.
In the above type of system, all information transfers were established on a master/slave basis, with the unit requesting and being granted access to the bus becoming the master and the unit being addressed by the master becoming the slave. Accordingly, when the system was powered up, the user established which system was to become the master and the other the slave. This in turn also established the procedure for initializing or clearing both systems (i.e., through the master unit).
In certain systems, it becomes difficult and impractical to establish a discipline or procedure by which one system initializes the other system. For example, in the case of two independently operated bus systems, each system bus could have units which could generate initialize or clear signals as part of a normal operating sequence (e.g. communication units). Thus, both clear signals could occur in close proximity to each other or in certain instances approximately at the same time.
It has been frequently found that the two system buses are interconnected so that the receiving circuit from one bus is coupled to the transmitting circuit of the other bus and the receiving circuit of the other bus is coupled to the transmitting circuit of the one bus. This arrangement creates a deadlock condition when it is powered on. To avoid this situation, some systems have used delay lines and delay line edge detector circuits for detecting which signal was first received. However, while this solves the situation when the signals occur in close proximity to each other, it does not overcome the instances where the signals arrive simultaneously. The result is that the circuits tend to oscillate at the delay line circuit frequency.
Accordingly, it is object of the present invention to provide an improved arrangement for interconnecting two independently operated bus systems.
It is a further object of the present invention to provide bus interconnection apparatus which bidirectionallly transmits control signals in a reliable manner.